Photochemical Dimerization of Dibenzylideneacetone. A Convenient Exercise in [2+2] Cycloaddition Using Chemical Ionization Mass Spectrometry

Photochemical [2+2] cycloaddition reactions are geometrically feasible and give rise to four membered rings. We outline a simple laboratory procedure for the photochemical dimerization of dibenzylideneacetone, a dienone, that can be easily prepared in the undergraduate laboratory. The dimerization is confirmed by chemical ionization mass spectrometry, and other spectroscopic techniques are used to establish the structure of the product. A second species can be isolated from the reaction. This is a trimer and is the result of three [2+2] cycloadditions of the dienone.